The present invention relates to an inverter circuit, and more particularly to an inverter circuit which operates stably irrespective of a great fluctuation in input voltage.
It is well known that switching elements such as thyristors, SCRs (Silicon Controlled Rectifiers), and the like (hereinafter referred to as thyristors) are adapted for use in an inverter circuit. There is a known prior art inverter circuit, for example, with the thyristor including a load which is connected in series to a reactor and a capacitor. This circuit provides for the supply of the load with a high-frequency power with the aid of the series resonance of the reactor and capacitor. The use of the series resonance of LC, however, causes the problem that the fluctuation of the load to a great impedance causes an excessive current to be produced and to flow into the thyristor with the result of the possible destruction thereof. Thus, such an inverter circuit of the series type has the drawback of a small allowance relative to the load fluctuation. It is, therefore, difficult to make use of the inverter circuit for a power supply circuit which supplies a power to a non-linear load such as an element with Zener characteristics having its impedance abruptly changed depending upon a voltage applied thereto.
A prior art circuit such as shown in FIG. 1 has been proposed to improve the series inverter circuit for application to a load with the great fluctuation. A DC supply 1 in FIG. 1 is, for the most part, a power supply obtained by rectifying a commercial AC supply.
In this circuit, thyristors 4 and 5 are alternately turned on and off to supply a load 8 with a high-frequency current A resulting from the resonance of capacitors 2 and 3 and a reactor 6 when the thyristor 4 is turned on, and to supply the load 8 with a high-frequency current B resulting from the resonance of the capacitors 2 and 3 and a reactor 7 when the thyristor 5 is turned on, thereby supplying the load 8 with a high-frequency power. For improvement against the above-mentioned drawback, the reactors 6 and 7 are provided with a secondary winding 9, which is connected to the power supply 1 through a diode 10. The fluctuation of the load 8 to a small impedance to increase the high-frequency currents A and B causes an increase of a voltage induced to the secondary winding provided at the reactors 6 and 7. The exceeding of the induced voltage over a voltage of the power supply 1 causes the diode 10 to be conductive. This forms a feedback circuit which feeds a portion of the high-frequency currents A and B flowing into the load 8 back to the power supply 1, and thus suppresses the increase in high-frequency current to prevent the destruction of the thyristors 4 and 5.
An increase in the voltage of the power supply 1 in the circuit of FIG. 1, however, requires an increase in the voltage induced in the secondary winding by so much in order to bring the feedback circuit of the secondary winding 9 and the diode 10 into operation. This results in the excessive high-frequency currents A and B.
If the power supply 1 is not formed by a cell, but obtained by rectifying the commercial AC supply without any voltage stabilizer, then the fluctuation of the output voltage from the power supply 1 is expected to be great. To prevent this, the number of turns of the secondary winding 9 may be increased to make the induced voltage great. In this case, however, the feedback circuit is disadvantageously operated even at a small voltage of the power supply 1, so that the load 8 cannot be supplied with a predetermined high-frequency power at the low supply voltage.
As mentioned above, in the conventional circuit as shown in FIG. 1, a stabilized constant voltage supply is needed to prevent the flow of the excessive current into the thyristor and to supply the load with the predetermined power. This, therefore, makes it difficult to make use of a power supply with a great fluctuation such as one which produces a pulsating voltage as obtained by rectifying the commercial power supply.